This invention relates to a charge coupled device and, in particular, a charge coupled device for reading signals from a solid-state image sensing device.
A charge coupled device as shown in FIGS. 1 and 2 is known in the art. FIG. 1 is a top view showing a conventional solid-state image sensing device and FIG. 2 is a cross-sectional view as taken along line I--I in FIG. 1. The device includes an n-type region 12 as a transfer channel and p.sup.+ -type regions 14A, 14B as a channel stopper in the surface area of a p-type semiconductor substrate 10. The n-type region 12 is formed between the p.sup.+ -type regions 14A and 14B. Regions 12, 14A and 14B are entirely covered with an insulating layer 18. The device includes electrodes 20A, 20B, . . . on insulating layer 18 to control an electric field in the transfer channel. The electrodes 20A, 20B, . . . extend in a direction perpendicular to that in which the transfer channel extends. The electrodes 20A, 20B, . . . are selectively connected to terminals .phi.1, . . . , .phi.4 where control voltages are received at a predetermined phase difference. That is, electrodes 20A and 20E are connected to terminal .phi.1 and electrodes 20B and 20F are connected to terminal .phi.2. Electrodes 20C and 20G are connected to terminal .phi.3 and electrodes 20D, 20H are connected to terminal .phi.4. When electrodes 20A, 20 B, . . . receive the corresponding control voltages sequentially, an electric charge in the transfer channel is transferred in the direction indicated by arrow A in FIG. 1.
FIG. 3 shows potential distribution in n-type region 12 and p.sup.+ -type regions 14A and 14B. When the control voltage is supplied to, for example, electrode 20D, potential 22 in the n-type region 12 is structurally set at a level lower than potentials 24A and 24B (i.e., a substrate potential level) in p.sup.+ -type regions 14A and 14B. The charge is transferred from the portion in n-type region 12 situated below electrode 20C to the portion in n-type region 12 situated below electrode, 2D. Potential 22 in the region falls to level 22A as shown in FIG. 3. This charge is indicated by a hatched area in FIG. 3, noting that a portion 26 in FIG. 3 shows a defect in the n-type region.
With the aforementioned charge coupled device, when control voltage is applied to, for example, electrode 20D, a corresponding charge is stored in a transfer channel at a portion below the electrode 20D. At this time, the charge so stored is uniformly distributed in the channel-width direction in which it is transferred, as indicated by an arrow A in FIG. 1. Where a larger charge, for example, is transferred, a greater potential change occurs at a portion stored with the charge. In other words, an adequately larger drift field is locally self-induced in the transfer channel in which case the charge is moved at high speed in the transfer channel. On the other hand, if a smaller amount of charge is involved for transfer, a very weak electric field is self-induced in the transfer channel and thus the electric charge is moved in the transfer channel at a very slow speed corresponding to the thermal diffusion. That is, the speed with which the electric charge is transferred in the charged coupled device is slowed down due to a decrease in the amount of charge to be transferred. When the transfer speed needs to be set above a predetermined value, there is a possibility that an adequate dynamic range will not be obtained in the charge coupled device. Where a defect occurs in the transfer channel as at a portion in FIG. 3, a predetermined amount of charge is trapped as transfer loss for each transfer. The transfer inefficiency .epsilon. of the charge coupled device is represented by: EQU .epsilon.=QT/QS
where
QT stands for the amount of charge trapped. PA1 QS stands for the amount of charge to be transferred.
The transfer inefficiency is increased in proportion to the amount of charge to be trapped and in reverse proportion to the amount of charge to be transferred. When the charge is transferred as a signal, such trapping causes a lowering in the reliability of the device.